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Discussion Starter · #1 ·
I have been working on a new frame and have welded in the chainstays and seatstays. I am still having trouble getting everything to clear each other in the chainstay/chainrings/tire area and thought I was giving myself enough space this time but I did not take into account giving clearance for crank arms. The crank arms to not quite hit the chainstays but do get very close. I have a little dimpling setup I have been using for the chainring clearance but that seems like a non-ideal method for the crank arms. I used stays with a single bend in them and I feel if I replace them with double bend one I can gain the clearance I need. I am wondering if cutting out and welding in new chainstays would be a way to go or if it would be causing too much warping in the BB and across the back end as I already have the seatstays in there. I am using some single speed sliding drops from Bike Fab Supply. Are there any other considerations for welding in new tubes?
 

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I explored this by leaving out BB spacers and artificially making pinch bolt cranks narrower. I needed >1mm of space between the chainstay and crank arm to avoid contact. I don't have an explanation for why so little clearance was necessary. I'm a big guy and very good at executing 50% of a tailwhip, so i expected to need a lot more than that. I bet you're fine.

Alternatively you could

-ovalize the chainstays at the potential contact point
-grind the inside tip off the cranks
-run a slightly longer BB spindle
-run shorter cranks


You could certainly weld in new chainstays, but i wouldn't be in a hurry to do it. It's a bummer the seat stays are already in place, too. If your heat control isn't great and you used a lightweight BB shell i wouldn't consider it.
 

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I recently built a frame where I didn't allow enough clearance between crank and chainstay. First I tried dimpling but it wasn't enough to account for sideload bending.

I just cut them out. I did one side a at time to keep to all in alignment. I could have just done the drive side but I wanted to keep them looking the same as i was moving from 7/8 round to 3/4 round on the new ones. Keep your heat in control and you should be fine.
 

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You guys have me wondering why i was able to get my cranks so close to the chainstays... the frame i was experimenting with used these stays. Pretty unremarkable.
 

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There are 2 lines of thought in operation here:

1: The rear end will flex and your crank will hit the chainstay.
2: The rear end won't flex and therefore you don't need to worry.

Since you have an advanced build here, I would be inclined to not do anything dramatic, assemble the frame up in the 'metal' and test it out. If you have a strike, dimple it until you get a clearance. If you don't have an issue, well, you proved to yourself the rear is stiff enough and the clearance is ok.

As you will have found, if ok, paint the frame. If not, go back to the original thought of replacing the stays. You will loose nothing by trying it out. You can plan # 2 frame with this issue well understood.

Eric
 

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I'm definitely no frame builder, but I see no problem with the clearance you have in those photos, especially considering you haven't installed a spacer behind the BB cups yet. I have a 2018 Kona UNIT and I'm running older M780 cranks with maybe a smidge more clearance than in your photos and I have no issues what so ever with rub/contact. As said, finish it up, put the spacers behind the BB cup(s) and go give it a test ride, then if you can't get it sorted or it just doesn't hit, then tackle that then.

This is a the crank/chainstay with a dimple. Those are some xtr cranks (narrowest I had) and no spacer behind the cup at the bb just to see what I would get.

View attachment 1267243 View attachment 1267245
 

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Discussion Starter · #11 ·
I decided to cut the old stays out and replace them with some new ones. I have not yet put in a BB and some cranks to check all the clearance areas but from measuring with calipers I seem to have gained the clearance from crank arm to stay. I may have lost a bit of space on the chainring clearance it seems.
This is my third frame that I have done so any other helpful pointers would be awesome. This forum has been a wealth of information.
I have also wondered the efficacy of seat stay bridges. I have seen that some builders don't add them for disc brake frames and other use them to add a bit of flair. Are they really the most necessary? The bridge is probably the biggest PITA for me to weld and I always make at least a few holes in the process.
 

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